1. Field of the Invention
The present invention relates to a voltage controlled oscillator which utilizes the resonance of parallel-LC tank circuits and more particularly to a voltage controlled oscillator that is preferably used as a local oscillator in a phase locked loop circuit.
2. Description of the Related Art
Conventionally, ring-voltage controlled oscillators (R-VCO) have been used as a local oscillator (LO) in a phase locked loop (PLL) circuit that is employed for frequency multiplication and phase synchronization. The R-VCO has an odd number of CMOS (Complementary Metal Oxide Semiconductor) inverters that are annually coupled to each other. This configuration provides an advantage that the R-VCO can be incorporated into a MOS integrated circuit. However, the R-VCO has disadvantages of high jitter and high phase noise.
On the other hand, the voltage controlled oscillator utilizing the resonance of a parallel-LC tank circuit (LC-VCO) has been recently used as a local oscillator. The LC-VCO includes inductors and variable capacitors which are coupled in parallel to each other to form a parallel-LC tank circuit. The resonance of the parallel-LC tank circuit causes an AC signal to be delivered at a resonant frequency. A resonant frequency is a frequency at which the reactance of a parallel-LC tank circuit is infinite, and the resonance refers to a phenomenon in which a current flows alternately through the inductors and variable capacitors in a parallel-LC tank circuit. The variable capacitor is a varactor element or the like, the capacitance of which varies with a control voltage applied thereto. The capacitance of the variable capacitor is adjusted to thereby control the frequency of the oscillating AC signal. For example, such an LC-VCO is disclosed in the literature, Ali Hajimira and Thomas H. Lee, “Design Issues in CMOS Differential LC Oscillators”, IEEE JOURNAL OF SOLID-STATE CIRCUIT, Vol. 34, No. 5 (MAY 1999).
The LC-VCO has the following advantages over the R-VCO. First, the LC-VCO has a lower noise level as compared with the R-VCO. This feature derives from the fact that the LC-VCO includes a less number of transistors which may cause noise because it is primarily based on the resonance of a parallel-LC tank circuit. This feature thus allows the LC-VCO to be preferably incorporated into high-speed optical telecommunications devices, cellular phones, wireless LANs or the like.
Secondly, the LC-VCO can easily provide higher oscillation frequencies than the R-VCO. This is because the LC-VCO is primarily based on the resonance of a parallel-LC tank circuit, whereas the R-VCO is made up of transistors and utilizes their logic gate delays.
Thirdly, the LC-VCO has a smaller range of variations in oscillation frequency for a control voltage as compared with the R-VCO. This feature allows for a lower tuning sensitivity and less variations in oscillation frequency caused by variations in control voltage, resulting in low noise.
FIG. 1 is a circuit diagram showing a conventional LC-VCO, and FIG. 2 is a plan view showing the conventional LC-VCO. As shown in FIG. 1, the conventional LC-VCO 101 is connected to a supply potential line VCC and a ground potential line GND. An inductor section 2, a variable capacitor section 3, a negative resistance section 4, and a current regulation section 5 are connected with one another in that order from the supply potential line VCC towards the ground potential line GND in the LC-VCO 101.
The inductor section 2 is provided with two spiral inductors 6a and 6b. Ends of the spiral inductor 6a and 6b are connected to the supply potential line VCC, with the other ends being connected to output terminals 7a and 7b, respectively.
The variable capacitor section 3 is provided with two varactor elements 8a and 8b. One end of the varactor element 8a, e.g., a well electrode is connected to the output terminal 7a, while one end of the varactor element 8b, e.g., a well electrode is connected to the output terminal 7b. The varactor elements 8a and 8b are connected to each other at the other ends thereof, e.g., the gate electrodes, to which a control voltage is applied.
The negative resistance section 4 is provided with N-channel transistors 9a and 9b. The N-channel transistor 9a has the drain connected to the output terminal 7a and the gate connected to the output terminal 7b. On the other hand, the N-channel transistor 9b has the drain connected to the output terminal 7b and the gate connected to the output terminal 7a. 
The current regulation section 5 is provided with an N-channel transistor 10, with the drain of the N-channel transistor 10 connected to the sources of the N-channel transistor 9a and the N-channel transistor 9b. Additionally, the N-channel transistor 10 has the source connected to the ground potential line GND and the gate to which a bias voltage is applied.
Now, a layout of the conventional LC-VCO 101 will be described below. As shown in FIG. 2, the LC-VCO 101 is provided in a semiconductor integrated circuit device in which multilayer interconnection layer 12 are deposited on a semiconductor substrate (not shown). The spiral inductors 6a and 6b are deposited in the topmost layers of the multilayer interconnection layer 12, respectively. A varactor element formed region in which the varactor elements 8a and 8b are to be formed and a transistor formed region in which the N-channel transistors 9a, 9b, and 10 are to be formed are located out of the underlying region of the spiral inductors 6a and 6b on the surface of the semiconductor substrate. On the other hand, no other elements nor conductors such as wirings are formed in the underlying region of the spiral inductors 6a and 6b. 
In this manner, the conventional LC-VCO layout is designed such that the spiral inductors are deposited in the topmost layers of the multilayer interconnection layer, with no elements such as varactor elements or transistors nor conductors being deposited in the underlying region of the spiral inductors. This design is intended to prevent magnetic fields created in the spiral inductors from having an adverse effect on the operation of active elements or from inducing a current flowing through conductors to cause power loss. Such an LC-VCO having a layout of this type is disclosed in the aforementioned literature and Japanese Patent Laid-Open Publication No. 2002-9299.
However, the aforementioned prior art technique has the following problems. That is, the conventional LC-VCO has a larger layout area than the R-VCO. For example, the R-VCO may have a layout of a rectangular area with a 75 μm vertical side and a 150 μm horizontal side, whereas the conventional LC-VCO 101 has a layout of a rectangular area with a 250 μm vertical side and a 300 μm horizontal side, being about seven times the R-VCO in layout area.